The invention relates to a balancing circuit for voltages of a series connection of capacitors, particularly for intermediate circuit capacitors of a frequency converter, there being at least two intermediate circuit capacitors connected in series over intermediate circuit voltage.
Series-connected electrolyte capacitors are usually used as the energy reserve of the DC side in frequency converters. The number of capacitors to be connected in series depends on the supply voltage of the frequency converter, being usually one capacitor for 230 volts, two for 400 to 500 volts, three for 690 volts and four for 1000 volts. Series connections of capacitors can also be connected in parallel in an intermediate circuit. The number of parallel connections depends on the output current of the frequency converter.
The leakage currents of capacitors typically differ from one another, which means that the supply voltage of the static state acting over the series connection is not divided evenly between the capacitors. This may result in a situation where a single capacitor is subjected to a voltage which exceeds the allowed limit in the dynamic state due to the influence of current ripple and capacitance tolerances, for example. For this reason, ‘balancing resistors’ are usually connected in parallel with the capacitors, the current flowing through the resistors being much higher than the leakage current of the capacitors. In that case the voltage distribution in the static state is mainly determined by the resistance ratios of the resistors. It is also known in the art to use active components in addition to the resistors, e.g. emitter follower connections, which provide stricter restriction without an unreasonable increase in the power loss. However, the use of active components increases the component costs.
Typical balancing resistance for one capacitor in a frequency converter of 100 kVA is 22 kilo ohms, its power dissipation being 5.2 watts with 500 volts, for example. Since with this voltage there are two capacitors and resistors in series, the total power dissipation is 10.4 watts. With higher voltages the power dissipation is naturally even greater.
To operate the frequency converter needs a certain amount of auxiliary power for control circuits and gate drivers. This power is typically 10 to 20 watts in the case of a frequency converter of 100 kVA. It is easy to note that the amount of waste heat produced in the balancing resistors is nearly equal to the amount of auxiliary power needed by the whole frequency converter. Thus it would be highly advantageous if the power dissipation required by balancing of capacitors could be utilized as the auxiliary power of the device.